Mendes Pimentel Pegmatitic Swarm

AMBLYGONITE - MONTEBRASITES FROM DIVINO DAS
LARANJEIRAS - MENDES PIMENTEL PEGMATITIC SWARM,
MINAS GERAIS, BRAZIL. II. MINERALOGY
Ricardo SCHOLZ1, Joachim KARFUNKEL2, Vladimir BERMANEC3, Geraldo Magela
da COSTA4, Adolf Heirich HORN5, Luiz Antônio Cruz SOUZA6 & Essaid BILAL7
1
Departamento de Geologia, Instituto de Geociências, Programa de Pós-Graduação em
Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brasil –
[email protected]
2
Departamento de Geologia, Instituto de Geociências, Universidade Federal de Minas Gerais,
Belo Horizonte, MG, Brasil – [email protected]
3
Mineralogy and Petrology Institute, Faculty of Sciences and Mathematics, University of
Zagreb, Zagreb, Croatia - [email protected]
4
Departamento de Química, Instituto de Ciências Exatas e Biológicas, Universidade Federal de
Ouro Preto, Ouro Preto, MG, Brasil - [email protected]
5
Departamento de Geologia, Instituto de Geociências, Centro de Pesquisa Professor Manoel
Teixeira da Costa, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brasil –
[email protected]
6
Escola de Belas Artes, Centro de Conservação e Restauração, Universidade Federal de
Minas Gerais, Belo Horizonte, MG, Brasil – [email protected]
7
Ecole Nationale Supérieure des Mines de Saint Etienne, SPIN, Instituto Héliopolis,
[email protected]
EXPERIMENTAL METHODS
A total of 72 samples, including primary and secondary phosphates, have been analyzed.
The X-ray diffraction were carried out at the Instituto de Ciências Exatas e Biológicas,
Universidade Federal de Ouro Preto, utilizing a Shimadzu model XRd 6000, with a Co tube and
an iron filter and scanning velocities of 0.5/min. Lattice parameters have been calculated with
the more intense reflections after the subtraction of the background and the K 2.
Several samples were studied with the SEM at the Microanalysis Laboratory of the
Universidade Federal de Minas Gerais, with a JEOL-JSM840A under varying current and
tension conditions.
The infrared spectroscopical analysis (FTIR) have been carried out at the infrared
laboratory of the Centro de Conservação e Restauração, Escola de Belas Artes, Universidade
Federal de Minas Gerais with powder samples at a BOMEM/HARTMANN & BRAUN
spectrometer, model MB100C23, with a diamond cell for micro beam, SPG46G model.
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Collected spectrums covered the range of 4000cm-1 – 400 cm-1 with a 4 cm-1 resolution, and have
been interpreted by using the Win-Bomen Easy, 3.01c version.
Chemical analysis with the EPMA were made at the Microanalysis Laboratory of the
Universidade Federal de Minas Gerais with an JEOL-JXA8900R in the EDS and WDS modes,
under the following conditions: acceleration tension  15 kV, current on the sample  2.00x10-8
Amps.
RESULTS
Today there are approximately 20 explored pegmatite bodies in the studied area;
amblygonite-montebrasites have been detected in 5 of them as primary phosphates (Fig. 1): CF –
Córrego Frio, JF – João Firmino, TE – Telírio, PO – Pomarolli, AF- Afrânio and JL – Jove
Louriano. These primary minerals occur together with quartz, muscovite and microcline,
sometimes with triphylite too. Crystals of amblygonite-montebrasites show a well developed
habitus and are of greenish color, sometimes creme or colorless. Once in a while small
completely transparent pieces in gem quality are seen in local markets.
Secondary amblygonite-montebrasites fill partially and/or totally substitution bodies.
However, they can occur at body walls too, usually as massive blocks.
Taken in account mineralogical assemblages and their mode of occurrence in the
pegmatite body (Scholz et al. 2001), the amblygonite-montebrasites of Divino das Laranjeiras Mendes Pimentel could be divided in 3 types:
- Type I: amblygonite-montebrasites of primary origin, associated with other primary minerals,
found usually at the intermediate zone;
- Type II: Secondary amblygonite-montebrasites, occuring in substitution/alteration bodies. The
habitus is hard to identify due to Dissolutions parallel to cleavage planes and etched
and corroded surfaces (Fig. 1). They are accompanied by other minerals of secondary
origin, like fluorapatite, muscovite and albite, and are related to a metasomatic stage
of crystallization (Moore 1973);
- Type III: Are also of secondary origin similar to Type II, however their habitus is prismatic
and elongated according to the crystallographic c-axis. The mineralogical assamblage
is complex, and Moore (1973) related minerals of this type to a hydrothermal
crystallization phase.
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CK
Fig.1. Montebrasite (Mo) with dissolution marks parallel to cleavage planes, associated to coockeite (Ck).
Infrared Spectroscopy
Measured infrared spectras where obtained for 5 samples. In the interval 1150-1050 cm-1
the transmission band of the (PO 4 )3- anion can be observed, with weak to medium absorption
intensities, caused by assymetric vibration of the PO 4 tetrahedra. The stretching effect occur at
the interval of 1200-1150 cm-1. The Li-O bond contributes to a transmission below the 500 cm-1
region. Bending effect of the PO 4 , as well as the vibration of AlO 6 stretching, yield transmitance
bands in the 650-500 cm-1 interval.
Using the correlation curve established by Fransolet & Tarte (1977) between OH
frequencies ( OH in the region between 3400-3350 cm-1 and  OH in the 840-800 cm-1 region) and
the fluorine percentage in amblygonite-montebrasites , calculation of fluorine content in the
analyzed specimen were done.
Correlation curves are given by the equations:
 OH = (-4.06x + 3396.5) cm-1 (1)
 OH = (2.74x + 804.6) cm-1 (2)
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x = percentage of fluorine.
We used the  OH band and equation 2, due to minor average errors. The percentage
calculations of the fluorine show contents between 1.78% and 4.03%. Values of  OH frequency
and fluorine percentage are shown in Tab.1, together with the type characterization (I, II or III).
Table 1- Type of montebrasites, infrared values of  OH bands and the calculated fluorine percentage
(Scholz et al. 2001).
Sample
JF-28
JF-13
JF-15
JF-25
CF-01
Type
III
II
II
II
I
 OH /cm-1
809.5
810.2
810.6
810.8
816.4
Calculated
1.78
2.04
2.19
2.26
4.03
Frequence
F (%)
Chemical analytical data for montebrasites have shown values of fluorine between 0.07
and 4.91. The data are presented in Tab. 2. The Fig. 2 shows the relation between the fluorine
content and the origin of the montebrasite – primary, metasomatic or hydrothermal.
Table 2 - Chemical composition and the Type of montebrasites.
Type
III
III
III
III
III
III
III
II
II
I
I
I
Sam
- ple
TE 23
TE 26
TE 22
JF 33
JF12
AF01
JF 28
JF15
JF25
JL-01 CF01
PO01
F
0.07
0.22
0.23
0.33
0.81
1.11
1.98
2.25
2.45
3.32
3.86
4.91
FeO
0.04
0.07
0.07
0.04
0.03
0.2
0.08
0.03
0.0
0.02
0.02
0.04
K2O
0.00
0.01
0.00
0.00
0.00
0.1
0.00
0.00
0.0
0.00
0.00
0.01
Na 2
O
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.00
0.13
0.04
CaO
0.00
0.00
0.01
0.00
0.01
0.01
0.01
0.01
0.0
0.01
0.01
0.20
P 2 O 5 50.44 50.40 50.00 49.98 50.69 50.59 50.39 50.49 50.18 49.93 49.62 49.81
MnO
0.01
0.00
0.01
0.00
0.01
0.01
0.01
0.01
0.02
0.01
0.02
0.01
Al 2 O
34.11 34.23 34.12 34.06 34.52 34.09 34.08 34.33 34.20 34.18 34.28 34.18
3
Total
84.65 84.84 84.34 84.30 85.73 85.38 84.60 85.00 86.88 86.07 86.31 87.12
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Fig. 2. Relation between the fluorine content and the origin of the montebrasite.
References are given at the end of part III.
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